Improved implementation techniques/methods

The high level of experience feedback on heavy component replacement has highlighted several aspects (human and technical) for which improved techniques and methods are recommended.

3.4.1. Radiation and conventional safety for workers

In the manufacturer’s factory, the national regulations will apply. Special attention will be given for component implementation on-site. Compared to other outage maintenance operations, the heavy component replacement concentrates a large source of risks for personnel. In addition to the fact that heavy loads are handled (up to about 400 t), the following risk factors are present:

• Large labour concentrations (can number more than one hundred persons) present in a restricted area;

• Work at multiple levels in same area resulting in risk from any dropped tool or other object;

• Large range of different skills (machining, welding, mechanics, handling, NDT, chemistry);

• Temporary need for radiographic inspection and gamma sources;

• High temperature areas (weld preheating and stress relieving);

• Work in confined areas (pipe grinding, inspection for RCL and steam line, SG 2 pieces final girth weld);

• Work in a radiological environment.

In addition to, or within, the national regulations applicable for the implementation site, the project team should perform a specific security study to identify and address all the potential risks linked to the implemen-tation.

3.4.2. Hoisting and handling

Major experience feedback on heavy component hoisting and handling has generated a list of recommen-dations:

• Overall hoisting and handling to be contracted to a specialized company directly or preferably through the installation contract. Throughout the world, a reduced number of such companies have corresponding experience.

• The positioning of the main component on its support and the piping end adjustment for welding require very precise handling (accuracy better than 1 mm).

• Exhaustive preplanning decreases the number of main component transfers to the minimum required. This is for limiting the risks and assuming a final optimal orientation while entering the containment building, where orientation changes are not easy. The shipment saddles should be used to allow transfers, and thus have the adapted design.

• Most of the power plants have not kept the erecting load capabilities of the polar crane due to maintenance reasons. Crane beams and an additional trolley should be adapted and qualified.

• The polar crane clearance below the hook is generally too low to allow for vertical lifting of the main components (SG and pressurizer). Several options should be scrutinized, such as lifting through the crane beams, using a temporarily installed additional jack lift, or lowering the cubicle walls by concrete removal, or a combined lifting with two trolleys for a component ‘tilting in the air’, or lifting by an externally placed ultra heavy crane through temporary openings above each component location.

• The polar crane utilization should be scheduled for the entire outage duration, including other users.

• The replacement of PWR internals in one set requires specific handling with the transportation/storage cask.

It is worth having additional auxiliary lifting means introduced in the reactor building to reserve the polar crane utilization for heavy handling.

3.4.3. Containment building opening

Containment openings are required when it is not feasible to move the main components through the plant’s equipment hatch. As previously discussed, in some cases, SGs are handled in two parts: lower assembly, with the primary channel head, tube sheet, lower secondary shell and tube bundle; and the upper assembly, with the remainder of the secondary shell and internal moisture separation parts, main steam nozzle and main feedwater nozzle. The two piece process reduces the length of the component parts to be handled and allows replacement through the equipment hatch for some plants where the equipment hatch diameter is adequate and there is enough space inside the equipment hatch to handle the shorter components, but not a complete component.

In the case where the equipment hatch diameter is not sufficient or the space inside the containment is not sufficient for the heavy component handling without excessive cost for removal and reinstallation of multiple items, a temporary opening through the containment building may be the preferred.

Nevertheless, it should be noted that the containment opening may not be feasible, depending of the containment structure design (e.g. post-tensioned concrete with cables directly in contact with concrete).

Containment openings have been located in the vertical cylindrical wall and in the dome shaped roof. The choice depends on access inside and outside the containment, interferences and available space and cost of a crane capable of handling the heavy load and having the long reach needed to lift a complete component over the top of the containment.

The containment opening is a significant addition to the scope of work for the project. The type of containment has the largest effect on the amount of work involved and the cost of the opening. Post-tensioned concrete greatly increases both the design and implementation effort for temporary openings.

For all cases, engineering must consider loads during the project, including all conditions related to component movement on the structure with the opening in place and the capability of the restored structure to meet design basis criteria for the remainder of the plant’s life. A number of processes have been successfully used for removal of the concrete. Key considerations are:

• Speed of removal;

• Capability to expose reinforcing steel (rebar) around the periphery of the opening for splicing without damage to the rebar;

• Removal of concrete from the liner plate and attached embedment without damage to the liner plate;

• Cost and environmental factors (dust, debris and water control).

A high pressure water jet has been used to remove concrete without damaging rebar or the liner plate.

While this method is more expensive, the benefits of eliminating damage to the liner plate and rebar and lower risk of delays in removing concrete for rebar splicing have made this the preferred choice for a number of cases.

Diamond wire sawing and robotic hammers are also effective tools that may be the preferred choice for specific situations.

Post-tensioned structures require additional engineering attention due to the non-linear effects of partial de-tensioning of the structure and re-tensioning after the opening is restored. The partially de-tensioned structure assumes a slightly distorted shape that results in the restored opening assuming the same distorted shape and slightly different loads internal to the structure when it is re-tensioned. In addition, the properties of the concrete in the opening are more critical due to the effects of shrinkage and creep on long term tendon forces. The post-tensioning tendons removed from the opening area are often replaced and this material has a long lead time. The sequence of replacing the liner plate, splicing rebar, installing tendon ducts, installing forms for the concrete, placing and curing the concrete and tensioning the tendons is, typically, the critical path for the project and plant outage.

3.4.4. Metrology and topometry

Use of precision measurements is strongly recommended for the replacement operation. In the first step they are used to confirm and update the as built documentation, with regard to component sizing, positioning and the piping and support interfaces. A 3-D as built modelling is obtained during the walk downs. In some cases, the collected data will be used for new component design purposes and then plant walk down will be implemented accordingly.

Specific attention is needed regarding the orientation of the components and interfaces with supporting structures. Typically, the components are installed, the supports fitted to them and the remaining surrounding structures completed. For installation of replacement components, adjustment of the supports may not be feasible. The metrology should provide the information needed to design adequate clearance for replacement components and shimming or other adjustment as applicable for the final interface with the supports.

Metrological tools are also used to perform the sizing of the new component in shop, and determine the cut sections of the main and secondary piping as well as the pipe and nozzle edges preparatory to welding.

A good accuracy (0.1 mm) of these tools is needed to obtain the required pipe end positioning (about 1 mm). Among the acceptable processes are the optical measurement (using theodolites), the digital photogra-metry, the laser tracking and the laser scanning.

Preference is to be given to tooling requiring the shortest stay of operators in the areas of severe radio-logical environment and having minimum impact on the critical path of the implementation.

Potential suppliers for main component replacements have experienced various measurement systems, and generally use a mix of existing measurement tools associated with specifically developed software.

3.4.5. Machining and welding

Machining and welding are part of the base technologies for main component replacements. As they directly affect the integrity of the primary circuit, the corresponding processes, tooling and operators have to be qualified in accordance with the selected codes and regulations.

Machining (cutting and bevelling) processes and corresponding tooling have to address the following criteria:

• Waste reduction;

• Prevention of foreign materials entering the piping system;

• Precise execution.

The welding process and tooling should be performed at high speed and result in a high quality uniform deposit with guaranteed chemical and mechanical characteristics.

Secondary and auxiliary piping is generally welded manually. A large part of the auxiliary piping may be prefabricated outside the containment. Primary piping and the core shroud vessel are remote control welded.

Tooling and process have been specifically developed and qualified by the several potential suppliers for main components replacement implementation. Weld design will be adapted for pre-service inspection and future NDE.

Particular geometries or a high dose rate environment may require the use of robots. This has already been undertaken in RCL, CRDM welding and core shroud replacements.

3.4.6. Shielding and decontamination

In order to fulfil the as low as reasonably achievable (ALARA) programme, a shielding layout is performed, in addition to possible studies on water level optimization in the circuit and reactor pool (see for example the case for BWR core shroud replacement). The utility should also take precautions during the shutdown phase to minimize the dose rate.

The layout and corresponding volumes of temporary shielding are to be established by the utility (in a shielding plan) with the participation of the installer having experience in this aspect.

Flushing the circuit before the outage (and flushing the cut pipes with clean water) is not fully efficient with respect to the dose rate, as it is necessary to eliminate a part of the oxide skin on the inner surface of the pipes.

Several processes for decontamination of pipe ends or pipe spool pieces are available. The two main types are mechanical (sand blasting) and chemical decontamination. In both cases, specific care is taken to plug the circuit against chemical products or mechanical residues (foreign material exclusion programme implementation).

Decontamination operations are complex and managed by specialized subcontractors. They require a specific qualification in accordance with the selected codes and regulations, including the innocuousness demon-stration process.

3.4.7. Component transportation on-site

The component transportation on-site has to be managed by the project team in conjunction with the component supplier, generally in charge of shipment and delivery to the site, the installer, and the site management. This work can be subcontracted to the handling/hoisting supplier with restrictive conditions on the allowed areas of manner of transfer and component orientation. An engineering study will take account of the optimal mode of transportation considering any interference (e.g. high voltage cables, underground obstacles), including necessary civil work modifications and/or ground reinforcement.